1. Field of the Invention
The present invention relates to terminal equipment, and more particularly to communication terminal equipment connected to an integrated service digital network (ISDN) for data transfer to and from a destination terminal.
2. Related Background Art
One type of conventional communication terminal equipment is a facsimile apparatus for transferring image data.
A G3 facsimile apparatus (G3 FAX) connected to an ISDN network has a structure as shown in FIG. 1. Between a communication control unit 13 and a B channel control unit 15, there are provided a modem (V21, V27ter, V29) and a modem control unit 14 connected to a telephone network similar to those built in the G3 FAX.
This is because a D channel control unit 16 for executing a call connection control and call release control sets the bearer capability (BC), i.e. the capability to operate at a particular channel bandwidth, of the network used for the designation of bearer service to that of an audio mode (audible sound bandwidth).
U.S. patent application Ser. No. 276,931 filed on Nov. 28, 1988, which issued as U.S. Pat. No. 5,142,568 on Aug. 15, 1992, discloses a technique in which data is transmitted to the ISDN by coding data with a coding method used by the G3 FAX and modulating it with a modem.
Such a conventional technique, however, is associated with the following disadvantages. Namely, if a bearer capability of a network is set at an audio mode, a modem is required so that the speed of data communication is lowered. Furthermore, since the bearer capability is set at the audio mode, the frequency of transmission errors becomes high.
In view of this, with the above technique disclosed in U.S. patent application Ser. No. 276,931, coded data is directly transmitted to ISDN without using a modem.
In this case, however, if the bearer capability of the network is different from that of a destination terminal equipment, communication cannot be established.